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Discovery of Potent and Orally Bioavailable Macrocyclic Peptide− Peptoid Hybrid CXCR7 Modulators Markus Boehm,† Kevin Beaumont,† Rhys Jones,† Amit S. Kalgutkar,† Liying Zhang,† Karen Atkinson,‡ Guoyun Bai,‡ Janice A. Brown,‡ Heather Eng,‡ Gilles H. Goetz,‡ Brian R. Holder,‡,# Bhagyashree Khunte,‡ Sarah Lazzaro,‡ Chris Limberakis,‡ Sangwoo Ryu,‡ Michael J. Shapiro,‡,∇ Laurie Tylaska,‡ Jiangli Yan,‡,○ Rushia Turner,§,◆ Siegfried S. F. Leung,∥,⊥ Mahesh Ramaseshan,⊥ David A. Price,† Spiros Liras,† Matthew P. Jacobson,∥ David J. Earp,*,⊥ R. Scott Lokey,*,§ Alan M. Mathiowetz,† and Elnaz Menhaji-Klotz*,† †

Pfizer Worldwide Research & Development, Cambridge, Massachusetts 02139, United States Pfizer Worldwide Research & Development, Groton, Connecticut 06340, United States § Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, United States ∥ Department of Pharmaceutical Chemistry, University of California, San Francisco, California 94158, United States ⊥ Circle Pharma, South San Francisco, California 94080, United States ‡

S Supporting Information *

ABSTRACT: The chemokine receptor CXCR7 is an attractive target for a variety of diseases. While several small-molecule modulators of CXCR7 have been reported, peptidic macrocycles may provide advantages in terms of potency, selectivity, and reduced off-target activity. We produced a series of peptidic macrocycles that incorporate an N-linked peptoid functionality where the peptoid group enabled us to explore side-chain diversity well beyond that of natural amino acids. At the same time, theoretical calculations and experimental assays were used to track and reduce the polarity while closely monitoring the physicochemical properties. This strategy led to the discovery of macrocyclic peptide−peptoid hybrids with high CXCR7 binding affinities (Ki < 100 nM) and measurable passive permeability (Papp > 5 × 10−6 cm/s). Moreover, bioactive peptide 25 (Ki = 9 nM) achieved oral bioavailability of 18% in rats, which was commensurate with the observed plasma clearance values upon intravenous administration.

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its internalization.5,6 It has also been reported to act as a scavenging receptor for CXCL12, resulting in modulation of the CXCR4 chemokine axis.7 CXCR7 expression is tightly regulated and is enhanced during pathological processes such as cancer and inflammation.8 It has been shown that CXCR7 plays an essential role in the CXCL12/CXCR4-mediated transendothelial migration of cancer cells.9 Recently it was reported that CXCR7 modulation ameliorates atherosclerosis and promotes vascular remodeling through increased cholesterol uptake in adipose tissue.10 CXCR7 has also been shown to act as a decoy receptor for adrenomedullin, a peptide hormone that plays a key role in cardiovascular development.11 A Phase II clinical study evaluated the safety and efficacy of CXCL12 gene therapy in heart failure patients,12 providing further impetus for the

INTRODUCTION The chemokine receptor CXCR7 is a member of the G-proteincoupled receptor (GPCR) family that binds to its chemokine ligands CXCL11 and CXCL12 with high affinity. CXCL12, also known as SDF-1, has many physiological effects, including acting as a stimulator of B cell lymphopoiesis and bone marrow myelopoiesis.1,2 The first identified CXCL12-binding chemokine GPCR was CXCR4, which received a great deal of attention because of its role in the HIV replication cycle, acting as a coreceptor for the entry of HIV into T cells.3 Further research has identified pathways activated by CXCR4 including extracellular signal-related kinase and protein kinase B leading to pleiotropic effects. Unlike CXCR4, CXCR7 signals solely through β-arrestin in response to CXCL12 binding, without noticeable activation of G-protein-coupled pathways.4 Nonetheless, the molecular pharmacology of CXCR7 is complex, as it is known to homodimerize as well as heterodimerize with CXCR4, affecting © 2017 American Chemical Society

Received: July 26, 2017 Published: October 18, 2017 9653

DOI: 10.1021/acs.jmedchem.7b01028 J. Med. Chem. 2017, 60, 9653−9663

Journal of Medicinal Chemistry

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Figure 1. Structures of peptide 1 and peptoid 2.

Macrocyclic hexapeptide 1 (Figure 1) has been reported as a CXCR7 modulator with functional activity in the low nanomolar range.44 Upon resynthesis, its functional activity in the CXCR7 β-arrestin assay was confirmed (EC50 = 46 nM), but the binding affinity against CXCR7 was relatively moderate (Ki = 2.1 μM). Furthermore, because of the presence of a basic arginine moiety, the compound showed poor permeability (Papp < 1 × 10−6 cm/s) in the MDCKII-LE permeability assay. In this work, we synthesized novel macrocyclic peptide−peptoid hybrids as an alternative to peptide 1 in order to identify compounds with improved binding affinity and lower overall polarity, with the goal of increasing the passive permeability toward the development of orally bioavailable CXCR7 modulators. To help guide the optimization of binding affinity, a homology model of CXCR7 based on the crystal structure of CXCR4 in complex with a macrocyclic peptide ligand45 was developed (see the Experimental Section). Induced-fit docking of peptide 1 provided a hypothesis for its bioactive conformation in the CXCR7 binding pocket (Figure 2) that

delivery of orally bioavailable CXCR7 ligands that can modulate levels of CXCL12 as an attractive therapeutic drug target. The identification and development of small molecules for CXCR7 have been described in several publications13−15 and patent applications.16−27 Since CXCR4 and CXCR7 share the same chemokine ligand, CXCL12, and peptide derivatives for CXCR4 have been successfully developed,28−31 there is also strong interest in identifying peptides to modulate CXCR7 activity.32,33 Peptidic macrocycles in particular may provide advantages over small molecules in terms of potency, selectivity, and reduced off-target activity.34,35

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RESULTS AND DISCUSSION Compound Synthesis. All of the peptides were synthesized via standard Fmoc solid-phase peptide synthesis methods using 2-chlorotrityl chloride (CTC) resin.36 After resin cleavage, the resulting acyclic peptide−peptoid hybrids underwent macrolactamization under dilute solution-phase conditions to deliver either the cyclic side-chain-protected peptide− peptoids or the final cyclic peptide−peptoids.37 The cyclic sidechain-protected peptide−peptoids were globally deprotected to afford the target compounds. All of the final compounds were purified using reversed-phase HPLC and analyzed to confirm >95% purity. CXCR7 Pharmacology. The binding affinities of peptides (given as Ki values) against CXCR7 were determined by their ability to displace radiolabeled 125I-CXCL12 from membranes obtained from a cell line overexpressing the human CXCR7 receptor.38 The functional agonist activities of peptides (given as EC50 values) against CXCR7 were determined by their ability to induce β-arrestin recruitment in a cell line overexpressing the human CXCR7 receptor.39 Passive Cell Permeability and EPSA. The rates of passive diffusion of peptides through cell membranes were determined in a permeability assay using a low-efflux Madin−Darby canine kidney (MDCKII-LE) cell line.40 To guide the improvement of passive permeability, the polarities of the peptides were measured by EPSA, a supercritical fluid chromatography method.41,42 EPSA values are analogous to calculated polar surface area (e.g., TPSA) values in medicinal chemistry programs. It has been shown that EPSA values of less than 90 are a necessary requirement for peptides to achieve measurable passive permeability.43 In contrast, while criteria for calculated PSA to achieve oral bioavailability have been described in the literature for small molecules (TPSA < 140), consistent guidelines for the prospective design of peptides or macrocycles have not been established.

Figure 2. Computational docking of peptide 1 in a homology model of the CXCR7 receptor. The TIC group is located in a large hydrophobic pocket (white) with room for further expansion. The arginine side chain points into a pocket containing acidic residues, forming favorable hydrogen-bonding and electrostatic interactions with negatively charged regions (red).

suggested strategies to improve the binding affinity. The proposed docking model shows the tetrahydroisoquinoline-3carboxylic acid (TIC) group residing in a large hydrophobic pocket formed by Trp100, Phe124, Leu128, Trp265, and Leu305 but without a tight contact for the TIC phenyl ring, 9654



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Table 1. Peptoid Variations with Improved Binding Affinity

a Values were determined by a radioligand binding assay and are reported as geometric means of at least two independent experiments with pKi ± SD in parentheses. bGlide XP docking scores, where more negative values indicate stronger binding affinity (see the Experimental Section).

affinities (Table 1), which indicates an improved complementary fit of the side chain into the hydrophobic pocket, resulting in enhanced binding of the peptoids. Notably, one of the most potent analogues, 6, also showed the best relative docking score in our computational modeling. Although it was not the most potent analogue, peptoid 4 was selected as the reference compound for the next set of structural modifications, with the knowledge that the ring substitutions leading to 5 and 6 could be reintroduced in the final design stage to regain binding affinity. In order to evaluate whether the backbone conformation of peptide−peptoid hybrid 4 remained unchanged compared to peptide 1, we employed NMR spectroscopy to gain information on the compound conformation in DMSO. The resulting data showed that 1 and 4 have similar conformations, both with two backbone transannular hydrogen bonds between 1-tryptophan and 4-arginine according to NMR temperature coefficient data (Table S1). The slightly larger 3JNH−αH value for 4 at 4-arginine (9.1 Hz) indicated that the H−N−Cα−H dihedral angle is closer to 180°. The overall backbone conformation is similar to those of previously reported orally bioavailable macrocyclic peptides,50,51 indicating the potential for this scaffold to provide reasonable passive cell membrane permeability and oral bioavailability when the side-chain polarity is limited. After obtaining peptoid analogues with good binding affinity, we shifted our attention toward improving the passive permeability. Our focus was on replacing the arginine side chain, which is positively charged at physiological pH and likely contributes to the poor passive permeability of the parent

indicating additional space for enhancing hydrophobic complementarity and improving potency. The implementation of peptoid residues offers the potential for modular diversification of macrocyclic peptides due to broad availability of synthetic building blocks.46−48 Peptide to peptoid substitutions have also been shown to improve both metabolic stability and cell permeability.49 Since the TIC group in 1 contains an N-benzyl substructure, we hypothesized that it could be replaced by an N-linked peptoid residue, allowing the efficient exploration of peptoid side chains at this position using primary amine reagents. This was confirmed by the synthesis of peptide−peptoid hybrid 2 (Figure 1 and Table 1) with a binding affinity (Ki = 4.5 μM) comparable to that of peptide 1 (Ki = 2.1 μM). In order to test the hypothesis that the N-benzyl peptoid group would bind in a hydrophobic pocket with additional space, a series of analogues with larger side chains at the peptoid position were synthesized, guided in part by docking of a larger series of synthetically available building blocks into the CXCR7 receptor homology model (Table 1). Analogue 4 with an N-phenylpropyl side chain (Ki = 96 nM) showed a 50-fold improved binding affinity over the shorter N-benzyl (2) and Nphenethyl (3) moieties. Further elongation of the side chain did not lead to additional affinity gain (data not shown). Optimization of the side-chain phenyl ring of 4 with F and Cl substituents further increased the affinity by a factor of 15, culminating in 5 and 6 with single-digit nanomolar binding affinities, approximately 750-fold more potent than the initial peptoid 2. The relative docking scores, expressing free energy of binding, correlated well with the experimental binding 9655



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those of analogues 7−12. However, presumably because of the lack of functional groups capable of forming hydrogen-bonding or electrostatic interactions with the negatively charged regions in the binding pocket (Figure 2), its binding affinity decreased 8-fold compared with 4. Since peptoid 11 exhibited the best balance of good potency, matching that of 4, and low polarity (EPSA = 116), it was selected as a starting point for further structural optimization. After successful replacement of the arginine side chain with heterocyclic ring systems, we tested the effect of replacing the two tryptophan residues with alternate aryl side chains (Table 3). Substitution of one tryptophan residue in 11 by homohomophenylalanine (hhPhe) resulted in 14, which showed an EPSA decrease from 116 to 104, but its permeability in the MDCKII-LE assay remained low (Papp = 0.9 × 10−6 cm/ s). Additional replacement of the second tryptophan residue with hhPhe (15) or homophenylalanine (hPhe) (16) further reduced the EPSA below the aforementioned threshold of less than 90, resulting for the first time in measurable passive permeability (Papp > 1 × 10−6 cm/s). Since 15 and 16 had slightly reduced binding affinity, we were particularly intrigued by peptoid 18 with a nonaromatic tert-butyl substitution, which retained the binding affinity of the tryptophan analogue (14). With its further reduced polarity (EPSA = 79), 18 also exhibited the highest passive permeability measured thus far in this series (Papp = 4.8 × 10−6 cm/s). Interestingly, a comparably low EPSA value for the equipotent peptoid 17 did not translate to measurable permeability (Papp = 0.7 × 10−6 cm/s). With 18 emerging as the most promising analogue, we embarked on further optimization of various side chains to obtain peptides with an improved balance between potency and passive permeability (Table 4). First, at the original arginine position, we replaced the 3-pyridyl group with a thiazole ring, for which we expected a modest potency boost from a known structure−activity relationship (SAR) (Table 2). This was confirmed with peptide 19 (Ki = 9 nM). The pyridyl to thiazole substitution resulted in only negligible changes in EPSA and permeability. In the next step, shortening of the side chain from hhPhe (19) to phenylalanine (20) improved the permeability (Papp = 6.6 × 10−6 cm/s), but the potency was decreased 50fold. Again taking advantage of an existing SAR (Table 1), we anticipated a potency boost by switching to a 2,4-difluorosubstituted N-phenylpropyl side chain at the peptoid position. Indeed, 21 showed a 20-fold increase in potency (Ki = 25 nM) in comparison with 20. The introduction of an additional 4fluoro substituent on the phenylalanine side chain (22) further increased the permeability (Papp = 7.6 × 10−6 cm/s). In an attempt to eliminate another aromatic group from the peptide, we also investigated aliphatic side chains (23, 24). Interestingly, the introduction of a second tert-butyl group in 24 showed the highest measured permeability in this series (Papp = 8.3 × 10−6 cm/s). However, peptide 25 provided the best overall balance between potency (Ki = 9 nM) and passive permeability (Papp = 6.2 × 10−6 cm/s) and was progressed toward examination of its in vivo oral absorption potential in rats. Measured in the CXCR7 β-arrestin functional assay, the activity of 25 (EC50 = 15 nM) showed a 3-fold improvement over the initial peptide 1 (EC50 = 46 nM). To summarize, our design strategy was to employ peptoids capable of probing a larger diversity of side chains to efficiently explore a large hydrophobic pocket in the CXCR7 binding site. Simultaneously, we monitored physicochemical properties such as polarity (as measured by EPSA) with the aim of identifying

peptide 1. The EPSA values for peptides 1−6 were determined to be in the range of 135 to 150, and not surprisingly, they exhibited poor passive permeability (Papp < 1 × 10−6 cm/s). The docking model of peptide 1 in the CXCR7 receptor homology model indicated that the arginine side chain points into a polar pocket containing aspartate and glutamate residues, forming favorable hydrogen-bonding and electrostatic interactions with Asp275 and Glu290 (Figure 2). We anticipated that replacement of the arginine side chain with nonbasic or less polar residues while retaining binding affinity would be challenging. Nevertheless, arginine replacements with improved membrane permeability have been described in the literature,52,53 and we set out to explore the effects of unnatural amino acids containing heterocyclic ring systems (Table 2). Table 2. Arginine Replacements with Decreased Polarity

a

Values were determined by a radioligand binding assay and are reported as geometric means of at least two independent experiments with pKi ± SD in parentheses. bValues were measured by a chromatographic method.41,43

Replacing the arginine side chain of 4 with various five- and six-membered heteroaryl rings led to a pronounced decrease in polarity, with EPSA values ranging from 116 to 127 for peptoid analogues 7−12, approximately 20−30 units lower than that for 4 (EPSA = 145). Encouragingly, several arginine replacements (7, 9−11) were equipotent or showed even greater binding affinity compared with 4. To explore the lowest achievable polarity for this side chain, we also prepared non-heteroaryl phenylalanine analogue 13, which had an EPSA value similar to 9656



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Table 3. Tryptophan Substitutions with Reduced Polarity and Increased Passive Permeability

Values were determined by a radioligand binding assay and are reported as geometric means of at least two independent experiments with pKi ± SD in parentheses. bValues were measured by a chromatographic method.41,43 cValues were determined by a permeability assay in a low-efflux Madin− Darby canine kidney cell line.40 a

passive permeability. Peptides outside this ClogP range did not exhibit substantial permeability in the MDCKII-LE assay (Figure 4, right). Apparently, not only is high polarity detrimental to passive permeability, but high lipophilicity seems to impede passive perfusion through the membrane. Similar findings of a parabolic correlation between a variety of ClogP and experimental Papp values were observed by other groups as well, claiming “islands of permeability” where only peptides with the right balance of polarity, lipophilicity, and solubility are likely to have measurable permeability.54,55 The equivalent analysis correlating passive permeability with calculated PSA (TPSA) instead of EPSA revealed that the cutoff criteria established in the literature (TPSA < 140) would not have been applicable for this series of macrocyclic peptides in a prospective design (Figure S1). In addition to optimization of the side chains, N-methylation of backbone amide groups has been shown to have a beneficial effect on permeability in cyclic peptide scaffolds.51,56−60 We examined the effect of N-methylation on selected peptides developed in this study (Table 5). We focused mainly on Nmethylation of the R2 and R3 backbone amides since analogues methylated at these positions are expected to have a similar low-dielectric backbone conformation compared to cyclic peptides that were previously shown to have good permeability.48,55 We found that N-methylation was generally beneficial to permeability when ClogP was in a favorable range, as described above. For example, peptide 17 (Table 3) had shown poor permeability (Papp = 0.7 × 10−6 cm/s) despite its low polarity value (EPSA = 89). The likely reason for that is

peptide−peptoid hybrids that were both potent and passively permeable across cell membranes. The overall progression of peptides synthesized in these iterative design cycles is shown in Figure 3. The initial peptides showed only weak binding affinity and high polarity with no measurable permeability. In the first design step, implementation of N-linked peptoids led to peptide−peptoid hybrids with improved binding affinity but relatively high EPSA values (Table 1). Next, replacement of the basic arginine side chain yielded compounds with lower polarity while retaining moderate activity (Table 2). Substitution of the two tryptophan residues yielded analogues with further reduced polarity (Table 3) and ultimately resulted in peptides with improved potency and passive permeability (Table 4). The peptides described herein are only a subset of a larger collection of analogues that were synthesized and tested during the iterative design cycle process. Having a large data set of related analogues with experimental data in hand, we were interested to learn of any relationships between physicochemical properties and experimental data to guide prospective peptide design efforts. The correlation between passive permeability (Papp), polarity (EPSA), and lipophilicity (ClogP) is shown in Figure 4. In agreement with previous findings,43 our data confirm that EPSA values of less than 90 are indeed necessary for peptides to achieve measurable passive permeability. However, satisfying this rule alone did not always result in permeable peptides (Figure 4, left). Our data suggest that peptides also need to fulfill a certain lipophilicity requirement, and with this particular scaffold they must have ClogP values between 7.5 and 10 in order to yield measurable 9657



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Table 4. Side-Chain Optimization with Improved Balance between Potency and Permeability


Figure 3. Optimization of peptides 1−25 as measured by binding affinity (CXCR7 Ki), polarity (EPSA), and in vitro passive permeability (Papp). Compounds presented in Tables 1−4 are grouped by shape. Permeability values of peptides were binned and are highlighted by color. Peptide 25 (*) was selected for further characterization.

lipophilicity (ClogP = 7.8 and 8.5, respectively) toward the optimal range necessary to achieve passive permeability. As a result (Table 5), we improved the low permeability of 17 with both its mono-N-methyl derivative 26 (Papp > 2.7 × 10−6 cm/s)

the ClogP value of 7.1, which is below the optimal range of 7.5 to 10 (Figure 4). N-Methylation of the R3 (26) and R2/R3 (27) backbone amides not only further decreased the polarity (EPSA = 86 and 83, respectively) but also slightly raised the 9658



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Figure 4. Correlation between passive permeability (Papp), polarity (EPSA), and lipophilicity (ClogP). Dashed lines indicate the EPSA threshold of less than 90 and a ClogP range from 7.5 to 10, which are necessary to achieve measurable permeability for peptides with this particular scaffold.

Table 5. N-Methylation of the Cyclic Peptide Scaffold To Improve the Permeability

Compd.

R2

R3

CXCR7 Ki [nM]a

EPSAb

ClogP

Papp [10−6 cm/s]c

17 26 27

H H Me

H Me Me

24 (7.6 ± 0.1) 20 (7.7 ± 0.3) 211 (6.7 ± 0.2)

89 86 83

7.1 7.8 8.5

0.7 2.7 4.0


Table 6. Pharmacokinetic Profile of Compound 25 in Ratsa CLpb [mL min−1 kg−1]

Vd,ssb [L/kg]

t1/2b [h]

AUCb [ng mL−1 h]

Cmaxc [ng/mL]

AUCc [ng mL−1 h]

F [%]

f u,p

136 ± 9

4.4 ± 0.3

0.95 ± 0.01

124 ± 9

97 ± 18

224 ± 30

18

0.011

a

Pharmacokinetic parameters were calculated from plasma concentration−time data in male Wistar Han rats (two animals per route) and are reported as mean ± SD values. bFollowing IV administration (1 mg/kg) formulated in DMSO/Cremophor/saline (5:5:90 v/v). cFollowing PO administration (10 mg/kg) as an emulsion in 10% Miglyol 812/Cremophor RH40/Capmul MCM (30:40:30 v/v) and 90% water (SEDDS formulation).

and bis-N-methyl derivative 27 (Papp = 4.0 × 10−6 cm/s). Ultimately, we decided not to pursue the N-methylation strategy further, as in most cases N-methylation at the R2 position also had a deleterious effect on the CXCR7 binding affinity. To assess whether oral bioavailability can be achieved for peptides with measurable passive permeability as determined in the MDCKII-LE assay, the pharmacokinetics of 25 was examined in rats. The intravenous (IV) and oral (PO) pharmacokinetic results are shown in Table 6. After IV dosing, peptide 25 exhibited a high plasma clearance (CLp) that exceeded the rat hepatic blood flow value (70 mL/min/kg).61 Consistent with this observation, incubation of 25 (1 μM) with NADPH-supplemented rat liver microsomes resulted in facile metabolic turnover leading to a short half-life (45 min) and a high apparent intrinsic clearance value (610 μL min−1 mg−1).

The finding that 25 was resistant toward metabolism (half-life > 2 h) in rat liver microsomes lacking NADPH cofactor strongly suggests a role for rat cytochrome P450 enzymes in the oxidative metabolism of 25 as a cause of its metabolic instability, which leads to observed high in vivo plasma clearance. The volume of distribution (Vd,ss) for 25 was considerably greater than the total body water volume (0.6 L/ kg), indicating extensive distribution into tissues. The corresponding half-life after IV administration was relatively short (95% purity under a variety of conditions: (a) an HP1090 system coupled to a Phenomenex C18 (2) (5 μm, 100 Å, 4.6 mm × 150 mm) reversed-phase HPLC column, eluting with a solvent gradient of A:C, where A = 0.1% TFA in water and C = 0.09% TFA in ACN/water (4:1) over 20 min at a flow rate of 1.0 mL/min; (b) a Waters LC/MS/ELSD (single quad MS) system coupled to an Acquity HSS T3 UPLC reversed-phase column (1.8 μm, 100 Å, 2.1 mm × 50 mm), eluting with 0.05% TFA in water/0.05% TFA in ACN (95:5 to 0:100 over 5 min then 0:100 for 2.5 min) at a flow rate of 0.7 mL/min; and (c) a Waters LC/MS/ELSD (single quad MS) system coupled to a SunFire C18 reversed-phase column (5 μm, 100 Å, 4.6 mm × 50 mm), eluting with 0.05% TFA in water/0.05% TFA in ACN (90:10 to 50:50 for 3.75 min, then 50:50 to 5:95 for 0.5 min, then 5:95 for 0.5 min) at a flow rate of 2.0 mL/min. Specific conditions, retention times, and UV purities (215 or 220 nm) for individual peptide−peptoid hybrids are provided in the Supporting Information. CXCR7 Radiolabeled Ligand Binding Assay. The binding affinities of test compounds for the human CXCR7 chemokine receptor were determined by their ability to displace 125I-CXCL12 (PerkinElmer) from membranes obtained from CHO-K1 cells overexpressing the human CXCR7 receptor (DiscoveRx). Ki values are reported as geometric means of at least two independent experiments, each of which was run in duplicate. Test compounds were serialized in 100% DMSO and spotted into 96-well plates (NBS). Total binding wells were spotted with diluent. Nonspecific wells were defined by the addition of a potent CXCR7 agonist. To each well of the plate was added 10 μL of 125I-CXCL12 with a final concentration of 100 pM, followed by the addition of 90 μL of membranes. Both the membranes and 125I-CXCL12 were diluted to the appropriate concentration in assay buffer (HBSS containing 10 mM HEPES and 0.2% BSA). After the addition of membranes, the plates were incubated at room temperature for 2 h (shaking). Reactions were terminated by rapid filtration through poly(ethylenimine) (0.3%)-treated 96-well GF/C Unifilter plates (PerkinElmer). Unbound ligand was removed by washing the filters with ice-cold wash buffer. Filters were allowed to dry thoroughly prior to the addition of Ready Safe scintillation fluid (PerkinElmer). The amount of bound 125I-CXCL12 was quantitated by reading plates on a Trilux luminometer (PerkinElmer). CXCR7 β-Arrestin Functional Assay. The agonist activities of test compounds were determined by their ability to induce β-arrestin recruitment in CHO-K1 cells overexpressing the human CXCR7 receptor (DiscoveRx). EC50 values are reported as geometric means of at least two independent experiments, each of which was run in duplicate. The receptor is fused with the small enzyme fragment ProLink and coexpressed in cells stably expressing a fusion protein of β-arrestin and the larger, N-terminal deletion mutant of β-gal (called enzyme acceptor or EA). Agonist activation of the receptor by test compounds causes binding of β-arrestin (EA) to the ProLink-tagged CXCR7, resulting in the formation of active β-gal enzyme. Enzyme activity is measured using chemiluminescent PathHunter detection reagents (DiscoveRx). Prior to the assay, cells were removed from culture flasks and plated at a density of 6000 viable cells per well in white solid-bottom 384-well plates. After the addition of cells, the plates were placed in a humidified 37 °C, 5% CO2 incubator for 18−24 h. Test compounds were serialized in 100% DMSO and diluted in assay buffer (HBSS containing 10 mM HEPES and 0.1% BSA) prior to their addition to the cell plates. Medium was removed from the cell plates, and 20 μL of compound or diluent was added to the appropriate wells of the plate. Following compound addition, cell plates were incubated at 37 °C in 5% CO2 for 30 min. After incubation, 20 μL of PathHunter detection reagents (DiscoveRx) was added to each well of the plate. The plates were incubated at 23 °C in the dark for 1 h prior to reading on an EnVision plate reader (PerkinElmer). MDCKII-LE Permeability Assay. Passive permeability was determined using a low-efflux Madin−Darby canine kidney (MDCKII-LE) cell line (Pfizer, Groton, CT). Cells were grown in

of 25 was 18%. The reason for achieving oral systemic exposure despite the hepatic-blood-flow-limited intrinsic clearance of 25 remains unclear at the present time. One possibility is the saturation of first-pass metabolic extraction by rat cytochrome P450 enzymes in the small intestine and the liver62 due to the 10-fold higher PO dose administered relative to the IV dose. When normalized against the rat plasma unbound fraction (f u,p),63 the free Cmax value of 25 after PO dosing (∼1.3 nM) was approximately 7-fold lower than the corresponding CXCR7 binding affinity (9 nM). In order to probe CXCR7 pharmacology in vivo, sustained unbound plasma concentrations several fold over the Ki value are desired, which currently would be unattainable with a reasonable PO dosing regimen of 25.

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CONCLUSION

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EXPERIMENTAL SECTION

In this study, we have demonstrated the successful optimization of low-affinity, highly polar, and nonpermeable peptides toward macrocyclic peptide−peptoid hybrids with improved CXCR7 binding affinity (Ki < 100 nM) and in vitro passive permeability in the MDCKII-LE assay (Papp > 5 × 10−6 cm/s), thereby achieving our goal to identify potent and cell-permeable CXCR7 modulators. Moreover, bioactive peptide 25 (Ki = 9 nM, EC50 = 15 nM) with measurable passive permeability (Papp = 6.2 × 10−6 cm/s) also demonstrated reasonable oral bioavailability in rats (F = 18%). A pragmatic next step to further increase the oral absorption of 25 would be to improve its metabolic stability while maintaining the CXCR7 potency and favorable passive permeability characteristics.

All procedures performed on animals in this study were in accordance with established guidelines and regulations and were reviewed and approved by the Pfizer Institutional Animal Care and Use Committee. Pfizer animal care facilities that supported this work are fully accredited by AAALAC International. Peptide Synthesis. Standard solid-phase peptide synthesis was used to synthesize the peptide−peptoid hybrids. Unless otherwise stated, the resin (100−200 or 200−400 mesh; degree of cross-linking: 1% DVB), Fmoc amino acids, and other starting materials were commercially available and used without further purification. After cleavage from the resin, the resulting acyclic peptides were then cyclized under dilute solution-phase conditions: (a) BOP, HOBt, DIPEA, DCM ([substrate] ∼ 6 mM), 16 h; (b) EDC, HOAt, DIPEA, ACN/DCM (1:1, 0.5 mg of linear peptide−peptoid compound/mL), 16 h. Full synthetic details for individual peptide−peptoid hybrids are provided in the Supporting Information. Purification of Peptides. Crude material was dissolved in a minimal amount of water and purified using multiple conditions: (a) a Waters 4000 system connected to a Phenomenex Luna C18 reversedphase HPLC column (10 μm, 100 Å, 25 mm × 200 mm), eluting with a solvent gradient A:B where A = 0.1% TFA in water and B = 0.1% TFA in ACN/water (4:1); (b) a Shimadzu LC-8A preparative HPLC system coupled to a Higgins C18 analytical column (5 μm, 100 Å, 20 mm × 250 mm), eluting with 0.1% TFA in water/0.1% TFA in ACN (40:60 to 0:100) over 30 min at a flow rate of 10 mL/min; (c) a Shimadzu LC-8A preparative HPLC system coupled to a Higgins C18 analytical column (5 μm, 100 Å, 20 mm × 250 mm), eluting with 0.1% TFA in water/0.1% TFA in ACN (10:90 to 0:100) over 30 min at a flow rate of 10 mL/min; and (d) a Waters system coupled to a SunFire C18 column (5 μm, 100 Å, 19 mm × 100 mm), eluting with 0.05% TFA in water/0.05% TFA in ACN (90:10 to 50:50 for 10.5 min, then 50:50 to 0:100 for 0.5 min, then 5:95 for 1 min) at a flow rate of 25 mL/min. Specific conditions for individual peptide−peptoid hybrids are provided in the Supporting Information. 9660



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minimum essential medium α-nucleosides (Life Technologies, Grand Island, NY) supplemented with 10% fetal bovine serum, 1% minimum nonessential amino acids solution, 1% GlutaMAX, and 1% penicillin− streptomycin prior to seeding into Millipore 96-well cell-culture insert plates (EMD Millipore Corporation). MDCKII-LE cells were cultured on the insets with 100 μL of medium per well on the apical side and 36 mL for all 96 wells on the basolateral side. Donor solutions were prepared from HBSS containing 20 mM HEPES (pH 6.5). Stock solutions of test compounds, prepared at 5 mM in DMSO, were used to prepare donor solutions of 6 μM compound. Receiver solutions were prepared from HBSS containing 20 mM HEPES and 0.4% (w/v) BSA (pH 7.4). Prior to the assay, the cell-culture medium was removed, and the cells were preincubated with HBSS for 10 min. To start the assay, 100 μL of donor solution and 300 μL of receiver solution were added to the apical and basolateral chambers, respectively. After 90 min incubations, aliquots were taken from the receiver chambers to determine the translocated amount of compound. Samples were taken from the donor chambers before and after incubation to determine the initial concentration (C0) and recovery values. An internal standard solution, 0.5 μg/mL CP-628374 (MW = 687) in 100% methanol, was added to the receiver and donor samples. The samples were analyzed by LC−MS/MS to determine the peak area for the test compound and the internal standard. All incubations were conducted in triplicate. Apparent permeability (Papp) values were calculated according to the following equation:

Papp =

the CXCR7 homology model to identify peptoid substitutions favorable for binding.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jmedchem.7b01028. NMR temperature coefficients, NH chemical shifts, and 3 JNH−αH values; EPSA method; correlation plot of passive permeability versus TPSA; synthetic procedures and analytical data of peptides 1−27; IV and oral PK concentration versus time curves for peptide 25 (PDF) Atomic coordinates of the CXCR7 homology model with computational docking of peptide 1 (PDB) SMILES strings and CXCR7 Ki, EPSA, and Papp data (CSV)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +1 617 551 3517. E-mail: elnaz.menhaji-klotz@pfizer. com. *Phone: +1 831 459 1307. E-mail: [email protected]. *Phone: +1 650 392 0363. E-mail: davidjearp@circlepharma. com.

dx /dt C0 × A

ORCID

Markus Boehm: 0000-0002-7025-3287 Gilles H. Goetz: 0000-0001-5316-1586 Matthew P. Jacobson: 0000-0001-6262-655X R. Scott Lokey: 0000-0001-9891-1248 Elnaz Menhaji-Klotz: 0000-0003-3299-9480

where dx is the amount of compound in the receiver compartment, dt is the incubation time, C0 is the initial concentration in the donor compartment, and A is the area of the filter of the transwell plate. Pharmacokinetics. Rat pharmacokinetics was determined in male Wistar Han rats sourced from Charles River and weighing between 250 and 281 g (n = 2/route) following single IV bolus (1 mg/kg) and PO (10 mg/kg) administration. For the IV dose, 25 was formulated in DMSO/Cremophor/saline (5:5:90 v/v) and administered at 2 mL/kg via the jugular vein cannula. In order to prevent poor aqueous solubility from confounding the oral bioavailability determination, the PO doses of 25 were administered in a self-emulsifying (SEDDS) formulation. Peptide 25 was presented as an emulsion in 10% Miglyol 812/Cremophor RH40/Capmul MCM (30:40:30 v/v) and 90% water (10 mL/kg). Doses were administered by gavage tube. At 2 min (IV only), 5 min, 15 min, 30 min, 1 h, 2 h, 4 h, 7 h, and 24 h post dose, blood samples were taken from previously implanted indwelling carotid artery catheters. Blood was centrifuged to produce plasma, which was stored frozen until bioanalysis. Bioanalysis of plasma samples was completed using specific HPLC−MS/MS methods with lower limits of detection for 25 equal to or below 1 ng/mL. Pharmacokinetic parameters were calculated using standard noncompartmental analysis. Molecular Modeling. The crystal structure of CXCR4 in complex with the cyclic peptide antagonist CVX15 (PDB code 3OE0) was used to develop a CXCR7 homology model. The protein sequence of CXCR7 was aligned to the CXCR4 structure using ClustalW. The homology model of CXCR7 was generated using Prime version 3.1 from Schrödinger. Conformational modeling of cyclic peptides was performed using PLOP with robust backbone sampling, and the predicted conformations were used as input for subsequent docking calculations. Docking of compounds into the CXCR7 homology model was conducted using Glide XP from Schrödinger (release 20162). The induced-fit docking (IFD) protocol, which uses Prime and Glide to induce adjustments in the receptor structure, was employed to investigate binding models of key compounds, such as peptide 1 and peptide−peptoid hybrids 2 and 4. To support optimization of the peptoid position for CXCR7 binding, CombiGlide from Schrödinger was utilized to enumerate virtual libraries of peptide−peptoid hybrids based on commercially available primary amines and to dock against

Present Addresses #

B.R.H.: Middletown, Connecticut 06457. M.J.S.: Niantic, Connecticut 06357. ○ J.Y.: MassBiologics, Boston, Massachusetts 02124. ◆ R.T.: Monterey Peninsula College, Monterey, California 93940. ∇

Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare the following competing financial interest(s): M.P.J. and R.S.L. are co-founders and advisors of Circle Pharma, and each holds an equity interest in the company.

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ACKNOWLEDGMENTS We thank the Chinese Peptide Company for peptide synthesis support. ABBREVIATIONS USED CXCR7, CXC chemokine receptor 7; CXCR4, CXC chemokine receptor 4; CXCL12, CXC chemokine ligand 12; EC50, half-maximal effective concentration; ELSD, evaporative light scattering detector; HBSS, Hank’s balanced salt solution; HPLC, high-performance liquid chromatography; IV, intravenous; Ki, binding affinity; MDCKII-LE, low-efflux Madin− Darby canine kidney cell line; MS, mass spectrometry; NMR, nuclear magnetic resonance; Papp, absorptive permeability; PO, oral; SAR, structure−activity relationship; SEDDS, selfemulsifying drug delivery system; UPLC, ultraperformance 9661



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liquid chromatography; UV, ultraviolet; ACN, acetonitrile; DCM, dichloromethane; DMSO, dimethyl sulfoxide; Fmoc, fluorenylmethyloxycarbonyl; hPhe, homophenylalanine; hhPhe, homohomophenylalanine; TFA, trifluoroacetic acid; TIC, tetrahydroisoquinoline-3-carboxylic acid

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